Channel all low lever scheduling calls through single function

The idea of this change is to only call Do***SyncSessionJob from single
function TryJob and have all other functions in class call it. The hope
is that maybe someday we'll call it asynchronously.

Previously it would look like this:
http://www.plantuml.com/plantuml/svg/ZP5HJiCm38RVSue-0jAkW0JRa8OGJCquGDQuRKPg5pUTbjkJfXfQrzBnvlZ_REUtfLFiaiLZygGFRUEsPAt1GWYPbQ1R2s-KAVlUwWAIHQ3EwPZPLBKTWYda34sPmdSBZRjHiS6ZiaOhPmZd9eXHAe9Ru9ouMe5L7dJKnC14JuLuPohWfR9shx8lyOckP7BEzlDsx9SA5VjNselrgIO4T4RPvy1IdgVWs8_13vJ0nz2L427h7AoPYL_C2VIO5fUlAAyeLEQcQ3dQbnup2KqJmewVmAE_srXdzueIiZ_ntsry3LTBcGOdDZv3_vvjuFGVxiNePZFcPbl5khk7Up7vmmjrtFai_W00

Now it will look like this:
http://www.plantuml.com/plantuml/svg/XP5HJiGW48RVEKKVrMIlOAAxQxB6wARK0x1bsg9Jg5FOBBUNqbHRsTGtcE-3uH-u5MIppNts83zSRuy6KUcQCLus81t2Yoa5lZfPGxO9rbIEbKFRuLBCeIBecno-7FJsdnC7VHQefB3AwBKEB2b5yGXK6MftWCA3Z8mjGenMM1Ug1TeAn9CeFzcxjWqPQt7iLv3V2Yt8FviJsryweq5R9V0nkAQgSh3aK_5DSw1pkDLe3UMGO4zaw4bTGAOsc-ycF83Yjx8puQvArOxYGBZN9OU-3yj4Z5dNX27q7yxu-D6pthw3T1O3kJBPEVxvZyM35qOozGLTZHhDcxjRTZNdZAOX9_WD
The change should be roughly equivalent to today's behavior.

BUG=259913

Review URL: https://codereview.chromium.org/68013007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235814 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235814